“Why did you bail out? And the bottom line is, I go fight the fight,” Mr Santorum said.

Mr Romney adopted the same tactic as during Saturday’s debate, largely refraining from making digs at his opponents – though he pointed to Jon Huntsman’s period as ambassador to China under Mr Obama.

Mr Huntsman replied on Sunday: “I was criticised last night by Governor Romney for putting my country first. He criticised me while he was out raising money. I want to be very clear. I will always put my country first.”

Mr Santorum said Mr Paul had “never really passed anything of any importance”.

“One of the reasons people like Congressman Paul is his economic plan. He’s never been able to accomplish any of that, has no track record of being able to work together. He’s been out there on the margins and has really been unsuccessful in working together with anybody to do anything.”

And when asked about President Obama’s patriotic credentials, Rick Perry said: “We have a president that’s a socialist. I don’t think our founding fathers wanted to make our country as socialist country.”